1. Field of the Invention
The present invention pertains to the field of AC-to-DC converters, and more particularly to power factor correctors.
2. Background
A wide variety of electrically powered devices rely upon AC power for operation. Such devices include, e.g., power supplies, computers, monitors, printers, and copiers. These devices must incorporate AC-to-DC converters, which in turn must comply with standards for harmonic reduction that are set by governmental agencies. In particular, the International European Electronics Committee (IEEC) has established rigorous ceilings for harmonics in AC-to-DC converters. Therefore, such converters may be required to be equipped with harmonic reduction circuits, or power factor correctors.
Harmonic reduction is typically achieved with a boost converter circuit that is independently controlled with its own control integrated circuit (IC). Boost converter circuits are generic to the art. Their function is to draw sinusoidal or near-sinusoidal current from an AC input power line and maintain a constant supply voltage to a switch mode power supply (SMPS), which is typically pulse width modulated (PWM). These and several other types of harmonic correction circuits are well known in the art.
Conventionally, the various ICs available on the market to perform the control function for boost converter circuits have been designed to regulate the output of the boost converter so as to provide a fixed bus voltage for the SMPS, regardless of the AC input line voltage. The switch within the boost converter is controlled by a separate control IC or discrete circuitry. Completely independently of the SMPS, the boost converter control circuit senses and regulates the voltage bus. For example, U.S. Pat. No. 5,568,041, which is fully incorporated herein by reference, teaches a power factor corrector circuit and method for electronic ballasts that functions by regulating the output of the boost converter. Other references such as, e.g., U.S. Pat. No. 5,115,347, which is fully incorporated herein by reference, teach a method of switching the boost converter off above a certain line voltage.
However, such conventional control devices increase the percentage of power processed by the power stage of the power factor corrector as the AC input voltage increases and are required to process the entire power of the load. This places a stringent power requirement and component stress on the power stage. For this reason such devices require expensive components that are capable of handling the requisite stress. Additionally, such devices impose a power-factor-correction efficiency cost of seven to twelve percent reduction in total system efficiency. Thus, there is a need for a device that reduces harmonics at low cost and high efficiency.